1. Field of the Invention
This invention relates to a method and apparatus for fluid transfer across a membrane.
2. Description of the Prior Art
Successful extracorporeal circulation has previously been obtained by using bubble oxygenators, disc oxygenators, screen type and filming type oxygenators. All of such oxygenation devices are dependent upon a blood-gas interface in which the blood and oxygen are in intimate contact in order to achieve the necessary gas exchange whereby oxygen is transferred into the blood and carbon dioxide and other waste products are removed from the blood.
It has been hypothesized that the blood-gas interface inherent in all such designs is reponsible in part for hemolysis and protein denaturization. Such systems which are dependent upon a direct blood-gas interface are limited to a relatively short term use and are rarely used beyond an eight hour period. Thus, a need arose for the gas transfer between blood and gas without direct contact.
Indirect blood-gas transfer has been attempted across a gas permeable membrane positioned between the blood and the gas in order to reduce or minimize trauma to the blood and extend the periods of use. However, such membrane oxygenator devices have an efficiency which deteriorates with time, and are much more expensive than the earlier bubble types mentioned and are thus not used in the majority of cases except when long term support is necessary. One of the chief causes for the deterioration of the efficiency of such gas transfer membrane type oxygenators is due to the varying differential pressure across the membrane which contributes to the build-up of contaminants such as water vapor along the membrane surface on the oxygen side of the membrane and the protein, plateletts and other cells that build-up on the blood side of the membrane.
In addition, most of the prior art membrane oxygenators have been based on a silicon rubber membrane. Silicon rubber was chosen because of its permeability to oxygen and carbon dioxide in thin sections. However, silicon rubber is very difficult to deal with as it tends to stick to itself and thus requires special handling techniques which increase the cost of such devices and it is very difficult to manufacture in thin section with complete integrity. Moreover, in many of the present membrane oxygenator systems used, the blood-gas interface is not totally eliminated in that there are various reservoirs utilized in the systems which have a blood-gas interface at the surface.